High temperature oxidation behavior and research status of modifications on improving high temperature oxidation resistance of titanium alloys and titanium aluminides: A review

Oxidation protection of γ-TiAl-based alloys – A review

Laves phases with their comparably simple crystal structure are very common intermetallic phases and can be formed from element combinations all over the periodic table resulting in a huge number of known examples. Even though this type of phases is known for almost 100 years, and although a lot of information on stability, structure, and properties has accumulated especially during the last about 20 years, systematic evaluation and rationalization of this information in particular as a function of the involved elements is often lacking. It is one of the two main goals of this review to summarize the knowledge for some selected respective topics with a certain focus on non-stoichiometric, i.e., non-ideal Laves phases. The second, central goal of the review is to give a systematic overview about the role of Laves phases in all kinds of materials for functional and structural applications. There is a surprisingly broad range of successful utilization of Laves phases in functional applications comprising Laves phases as hydrogen storage material (Hydraloy), as magneto-mechanical sensors and actuators (Terfenol), or for wear- and corrosion-resistant coatings in corrosive atmospheres and at high temperatures (Tribaloy), to name but a few. Regarding structural applications, there is a renewed interest in using Laves phases for creep-strengthening of high-temperature steels and new respective alloy design concepts were developed and successfully tested. Apart from steels, Laves phases also occur in various other kinds of structural materials sometimes effectively improving properties, but often also acting in a detrimental way.

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Laves phases: a review of their functional and structural applications and an improved fundamental understanding of stability and properties

The improvement of high temperature oxidation of Ti–50Al by sputtering Al film and subsequent interdiffusion treatment

This study systematically analyzes the experimental results of the resistance of various coatings on superalloy IN-738LC to high temperature oxidation and corrosion. Three different pack cementation processes are carried out: pack-aluminization process, two-step aluminization–chromization process and siliconization process (with or without TiB2 in the packing mixture). Rhodium or palladium is deposited on the superalloy IN-738LC specimen by electroplating prior to the pack-aluminization processes. NiAl and Ni2Al3 phases are the main constituents of the aluminized coatings. Single Cr-rich phase forms on the surface of aluminized–chromized coatings. The Ni2Si, Ni3Si and TiSi phases exist after simple siliconization while Ni2Si, Ni3Si, TiSi, TiSi2 and TiB phases form after siliconization with TiB2 in the packing mixture. Among the three different categories of pack cementation coatings, aluminized coatings have the best isothermal and cyclic oxidation resistance at high temperature. When the temperature is over 850 °C, aluminized–chromized coating layers lose their protection against oxidation due to the spalling or cracking of the oxide scales during the test for cyclic oxidation. Siliconized coatings are brittle and crack severely due to thermal shock. Therefore, siliconized coatings are not suitable for superalloy IN-738-LC. Hot corrosion tests were carried out at 900 °C, a temperature above the melting point of Na2SO4 (884 °C). All three categories of pack cementation coatings show degradation of the corrosion scales and internal attack. Nevertheless, the aluminized coatings were still protective under hot corrosion conditions. The ranking of high temperature oxidation and corrosion resistance of all pack cementation coatings, from best to worst, is as follows: Rh–Al, simple Al, Pd–Al, Si+TiB2, simple Si, Pd–Al–Cr, Rh–Al–Cr and simple Al–Cr samples.

Improvement of high temperature oxidation and corrosion resistance of superalloy IN-738LC by pack cementation

The halide-activated pack cementation method was utilized to deposit a suitable coating on TiAl to ensure its oxidation resistance. TiAl alloys were coated by two different pack cementation processes with simple aluminide coating and Cr-modified aluminide coating. The coatings formed on the TiAl alloyed with Nb and Cr were thicker and tougher than those formed on the non-alloyed TiAl. The TiAl3 layer formed on the TiAl alloyed with Nb and Cr had much better cyclic oxidation resistance than that formed on the non-alloyed TiAl. This is attributed to a change in the crystal structure of the TiAl3 from the brittle tetragonal DO22 to the ductile cubic LI2 by addition of a small amount of Cr and Nb. The Cr-modified aluminide coating on the TiAl alloyed with Nb and Cr exhibited excellent cyclic oxidation resistance due to formation of the Al67Ti25Cr8 phase as the coating layer.

A study on aluminide and Cr-modified aluminide coatings on TiAl alloys by pack cementation method

The high temperature oxidation behavior of TiAl alloys coated with Ti–Al–Cr alloys using magnetron sputtering method was studied. The results showed that Ti–Al–Cr coatings improved the oxidation resistance of TiAl alloys. The weight gains at 950°C for Ti–Al–Cr coatings are more than those at 1000°C. Oxide formed on the surface of Ti–50Al–15Cr coating after oxidation consisted of Al2O3, whereas oxide formed on the surface of Ti–50Al–10Cr coating after oxidation is composed of a mixture of a large portion of Al2O3 and a small portion of TiO2. The effect of Ti–Al–Cr coatings on the improvement of oxidation resistance of TiAl alloys results from thermodynamic and kinetics factors contributing to Cr effect.

Ying Yang

Papers

A study on aluminide and Cr-modified aluminide coatings on TiAl alloys by pack cementation method

effect of ti-al-cr coatings on the high temperature oxidation behavior of tial alloys